Flood Mitigation: Berms

September 7, 2023

There are a variety of different terms for a raised structure that acts as a barrier between two areas. The terms berm, dike (or dyke), and levee are often used interchangeably to describe such structures that are used for flood mitigation. The main difference between the terms is how each is built to mitigate flooding. Both berm and levee refer to raised embankments that are either manmade or the result of excavated land. Note that “berm” can be used for a landscape feature not related to water management.

For our purposes, we will use berm to describe the raised banks intended for water management, as this is common language used to describe this form of flood mitigation.

What is a berm?

A berm can be a manmade sediment barrier placed at the edge of a slope or a wall and built adjacent to a ditch to guard against potential flooding. Berms are placed in flood-prone areas to protect against erosion, run off and high water. Typically berms are made of compost, sand, mulch or gravel materials; their density enables them to slow down and retain flood waters [1].

How does this option help to mitigate the impacts of a flood?

Berms contain dense sediment materials that decrease water velocity, control flow rates, and absorb excess water in the event of a flood. Normally placed in communities or sites prone to flooding, berms act as a barrier further mitigating the impacts of flood water. Berms are an effective form of flood mitigation, so long as they are built large enough to handle flood flows. For this reason, berms are often coupled with other mitigation measures, such as dams and dry ponds.

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Are berms already built in Canada?

Berms are the most commonly used flood protection structures in Canada [2] and can be found in places ranging from the Lower Mainland of British Columbia [3] to Newfoundland [4].

What are the costs to build and maintain?

The cost of building a lateral berm ranges depending on size, use (transverse or lateral), and materials. For flood mitigation, transverse berms are typically larger and more expensive. In the 1980s, Drumheller built a system of berms throughout the city, which proved beneficial for all subsequent flooding, including the 2013 event. After that flood, Drumheller started an $81.5 million project to upgrade its berms and other flood mitigation work [5]. In Nova Scotia and New Brunswick, a proposal to upgrade existing berms to protect the inter-provincial land link has been estimated at between $189-$300 million [6].

How long does it take to build?

Depending on the size of the berm, construction times are quicker than other infrastructure options, such as dams and diversions (transverse structures). For example, berms protecting Calgary, Alberta have been completed since 2013, but the first of two possible flood mitigation dams has only recently started construction [7]. The second is still undergoing feasibility assessment [8], and construction, if the project proceeds, is years away.

Operations lifetime expectancy?

Typically, the primary concern for the lifetime expectancy of a berm is the river breaching or overtopping the berm. Therefore, these structures should be upgraded over time to accommodate higher river flows, to address settling, or if otherwise necessary. For example, Drumheller’s current project mentioned above came 30 years after initial construction.

What are the associated risks?

Berms built along smaller streams and rivers tend to increase the water’s velocity and cause downstream erosion and riverbank damage. Berms do interrupt the natural floodplain function because the water is confined. This creates significant erosion and subsequent berm failure, causing damage to property [9]. Somewhat counter-intuitively, berms can result in a net increase in properties at risk. This arises from the perception that the berm makes the protected area “safe” (as opposed to “safer”), and so additional development happens in the protected area. When the berm is overtopped or fails, the resulting damage is larger than if the berm had not been built [10]. Furthermore, water can be diverted into and then retained in the protected area by the failed berm [11], p.13.

What are the environmental and watershed impacts of berms?

The environmental impacts of berms include riverbank erosion, habitat destruction, and stream instability. These impacts are mainly caused by the increase in velocity that berms cause by limiting the room for water to flow.

Berms are built to contain flood waters and prevent riverbank breaches that could flood developments. Impacts on the surrounding watershed can be significant, due to high flow rates and subsequent erosion. The function of floodplains is also disrupted, because water is being confined to the stream, rather than allowing flow to expand across the floodplain.

Would a berm help in a drought?

Flood protection berms are intended to aid in flood situations by keeping high water out of certain areas. Therefore, they tend to be of little use in a drought.

Sources

Government of New Brunswick, n.d., Water and Sediment Control Basins. https://www2.gnb.ca/content/gnb/en/departments/10/agriculture/content/land_and_environment/environmental_sustainability/control_basins.html. Accessed 2023-05-08.
Government of Canada, 2013, Reducing flood damage. https://www.canada.ca/en/environment-climate-change/services/water-overview/quantity/reducing-flood-damage.html#dykes. Accessed 2023-05-08.
Government of British Columbia, n.d., Lower Mainland Dike Inventory Maps. https://www2.gov.bc.ca/gov/content/environment/air-land-water/water/drought-flooding-dikes-dams/integrated-flood-hazard-management/dike-management/flood-protection-structures/dike-inventory. Accessed 2023-05-08.
Government of Newfoundland and Labrador, 2022, Newfoundland and Labrador Flood Management Strategy. https://www.gov.nl.ca/ecc/files/Newfoundland-and-Labrador-Flood-Management-Strategy.pdf. Accessed 2023-05-08.
The Drumheller Mail, 2023, Province announces $27.3 million for Drumheller flood mitigation project in 2023 budget. https://www.drumhellermail.com/news/35371-province-announces-27-3-million-for-drumheller-flood-mitigation-project-in-2023-budget. Accessed 2023-05-08.
Canadian Press, 2022, Dikes to protect N.S. N.B. link from flooding expected to cost hundreds of millions. https://www.canadianunderwriter.ca/insurance/dikes-to-protect-n-s-n-b-link-from-flooding-expected-to-cost-hundreds-of-millions-1004218994/. Accessed 2023-05-08.
Government of Alberta, n.d., Springbank Off-stream Reservoir. https://www.alberta.ca/springbank-off-stream-reservoir.aspx. Accessed 202305-08.
Government of Alberta, n.d., Bow River Reservoir Options. https://www.alberta.ca/bow-river-reservoir-options.aspx. Accessed 2023-05-08.
Lake Champlain Committee, 2014, Lessons From the Floods – Bad News Berms. https://www.lakechamplaincommittee.org/learn/news/item/lessons-from-the-floods-bad-news-berms. Accessed 2023-05-08.
Irwin A., 2021, How flood protection can paradoxically put people at risk. https://ec.europa.eu/research-and-innovation/en/horizon-magazine/how-flood-protection-can-paradoxically-put-people-risk. Accessed 2023-05-08.
Fraser Basin Council, 2010, Environmental Protection in Flood Hazard Management: a guide for practitioners. https://www.fraserbasin.bc.ca/_Library/Water/report_flood_and_environmental_protection_2010.pdf. Accessed 2023-05-08.

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